Stability and relaxometric characterization of a manganase(II) based macrocyclic complex containing malonate pendant

Debretsion, Abraham Estifanos and Bunda, Szilvia and Lihi, Norbert and Garda, Zoltán and Van Doorslaer, Sabine and Kun, Emese and Csupász, Tibor and Tircsó, Gyula and Jakabné Tóth, Éva and Kálmán, Ferenc Krisztián (2025) Stability and relaxometric characterization of a manganase(II) based macrocyclic complex containing malonate pendant. DALTON TRANSACTIONS, 54 (27). pp. 10751-10760. ISSN 1477-9226

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Abstract

In this study, we report the thermodynamic, kinetic, relaxation and structural features of the Mn(II) complex formed with a newly synthetized O-pyclen ligand bearing a malonate pendant. The thermodynamic stability of [Mn(OPMMA)] is lower (pMn = 6.27) than that reported for the Mn(II) chelates of OPC2A2− (pMn = 8.69) and 3,9-PC2A2− ligands (pMn = 8.64), and it dissociates faster than those at physiological conditions (t1/2 = 2.1 min). This behavior can be related to the assymetric structure of the ligand and the presence of an unsubstituted N donor in the macrocycle. DFT calculations revealed a capped trigonal prismatic coordination geometry for [Mn(OPMMA)], involving the coordination of the macrocyclic donor atoms, the two carboxylates of the malonate pendant arm as well as an inner sphere water molecule to Mn(II). The relaxivity values of [Mn(OPMMA)] (r1p = 3.48 and r2p = 5.85 mM−1 s−1 at 20 MHz and 298 K, respectively) are slightly higher than those found for [Mn(OPC2A)] and [Mn(3,9-PC2A)]. The water exchange rate (k298ex = (22.1 ± 0.1) × 107 s−1) is 2–4-times higher than that of the [Mn(OPC2A)] and [Mn(3,9-PC2A)] complexes. The slightly higher rotational correlation time as compared to [Mn(OPC2A)] (τ298rH = 55 ± 3 ps vs. 40 ps) can be related to the less compact structure of [Mn(OPMMA)].

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